1. Field of the Invention
The present invention relates to a method and apparatus for forming through holes in a ceramic green sheet using a laser.
2. Description of the Related Art
Laminated electronic components are often manufactured by laminating a plurality of ceramic green sheets together. In order to permit electrical connection between the layers, through holes must be formed in the individual layers. Such through holes are conventionally formed by a punching process using a die and a pin. According to the conventional punching process, the dimensional precision of the die and the pin determines the precision of the through hole. However, this conventional process has several drawbacks.
When a through hole having a diameter of 100 micrometers or less must be formed, it is extremely difficult to achieve the desired precision. Also, since the die and the pin have a relatively short life and need to be periodically exchanged. Because the pin and die are expensive, this substantially increases the cost of using the conventional process. Another drawback of the conventional process is that different dies must be used for different types of electronic components. Since the exchange operation is complicated, this adds yet further to the cost of the conventional process.
As a result of these drawbacks, several methods for forming through holes with a laser beam have been proposed. Two such prior art methods, each of which enables accurate formation of through holes having a diameter of approximately 80 micrometers, use laser beams to form through holes in the ceramic green sheet. In accordance with these methods, a single shot of the laser forms a single through hole in the ceramic green sheet. To form a plurality of such through holes, both methods rely on relative movement between the laser beam and the ceramic green sheet. The relative movement ensures that the laser beam moves from point to point along the ceramic green sheet to cut the desired through holes. In the first of these methods, the ceramic green sheet is kept stationary while the laser beam is moved from point to point using a galvanomirror. In the second of these methods, the laser beam is kept stationary and the ceramic green sheet is moved. In both cases, each respective though hole is formed with a separate shot of the laser reducing the productivity of the method.
In both of these prior art methods, the diameter of the laser beam is equal to the desired diameter (approximately 80 micrometers) of the through holes making it possible to apply the laser beam directly to the ceramic green sheet without using a mask. However, this also creates potential problems because the shape of the through hole will be distorted if the laser beam is emitted obliquely.
To avoid these problems, a laser-beam punching method as shown in FIG. 8 has been proposed. According to this known laser-beam punching method, a mask 82 having a predetermined number of circular through hole transmissive portions 82a is disposed a significant distance from a ceramic green sheet 84 mounted on a table 87. A laser source 81 is used to emit a laser beam L, having a diameter larger than each through hole transmissive portion 82a, through a mask 82. As a result, the single laser beam Lxe2x80x2 is divided into a plurality of sub beams Lxe2x80x2 forming a predetermined pattern of light on the ceramic green sheet 84 by means of a converging lens 83. As a result of this structure, a plurality of generally circular through holes are simultaneously formed in an irradiated area Lc on the ceramic green sheet 84.
While overcoming the problems of the above described laser punching methods, this method has its own drawbacks. Because the mask 82 is located a relatively large distance from the ceramic green sheet 84, there will be distortions in the shape of the through holes 86 formed along the perimeter of the irradiation area Lc. See FIG. 9. Only the though hole 85 (defined by the sub beam Lxe2x80x2 passing through the center opening of the mask 82) formed in the center of the irradiation area Lc will be circular in shape. The peripheral through holes (formed by the sub beams Lxe2x80x2 passing through the peripheral openings 82a in mask 82) will be elliptical in shape having inferior roundness and a large error from the desired shape. Accordingly, in order to form through holes which are of a desired roundness, it has been necessary to either form each through hole with a separate shot of the laser beam (the diameter of the beam being equal to the diameter of the through hole) or to use an expensive lens which will optically avoid such distortions. The former solution increases processing cost. The latter solution increases the cost of the processing equipment.
It is an object of the present invention to provide a method and apparatus for forming a plurality of through holes having a desired shape in a ceramic green sheet, with high precision.
According to one aspect of the present invention, a through hole forming method comprises the steps of:
mounting a mask on a surface of a ceramic green sheet, said mask having a plurality of through hole transmissive portions; and
irradiating said mask with a laser beam so that through holes are formed in said ceramic green sheet by sections of said laser beam which pass through said through hole transmissive portions.
Preferably, the laser beam is applied while the ceramic green sheet is in motion. In one embodiment, the ceramic green sheet is continuously moved in a predetermined direction, and a laser beam is applied to the ceramic green sheet while the sheet is in motion.
According to another aspect of the present invention, the process comprises the steps of: mounting on a ceramic green sheet supported by a table a mask provided with predetermined through hole transmissive portions; and applying a pulsed laser beam emitted from a laser source to the mask by using a galvanomirror to reflect the pulsed laser beam so that through holes are punched in the ceramic green sheet by laser beams having passed through the through hole transmissive portions, in which the ceramic-green-sheet processing method repetitively applies the pulsed laser beam while changing the reflection angle of the galvanomirror to a predetermined direction.
In each ceramic-green-sheet processing method, the laser beam may be applied to the mask further provided with transmissive alignment marks so that through holes are punched in the ceramic green sheet, and positioning holes may be simultaneously punched by Laser beams having passed through the transmissive alignment marks.
According to a further aspect of the present invention, the foregoing object has been achieved through provision of a laser processing apparatus comprising: a laser source; a laser-source driving circuit for driving the laser source; a table for holding an object to be processed, the object having a mask disposed thereon; a table driving circuit for moving the table in a predetermined direction; a control circuit for sending control signals to the laser-source driving circuit and the table driving circuit; and a converging lens disposed between the laser source and the mask, provided for converging a laser beam emitted from the laser source.
According to a still further aspect of the present invention, the foregoing object has been achieved through provision of a laser processing apparatus comprising: a laser source; a laser-source driving circuit for driving the laser source; a table for holding an object to be processed, the object having a mask disposed thereon; a galvanomirror directed to the table; a galvanomirror driving circuit for changing the reflection angle of the galvanomirror; a control circuit for sending control signals to the laser-source driving circuit and the galvanomirror driving circuit; and a converging lens disposed between the laser source and the mask, provided for converging a laser beam emitted from the laser source.
As is apparent from the foregoing description, according to the present invention, a mask provided with predetermined through hole transmissive portions is mounted on a ceramic green sheet. Thus, through holes having a shape substantially identical to that of the through hole transmissive portions are formed in the ceramic green sheet, and laser beams, having passed through the through hole transmissive portions in the periphery of the mask, simultaneously form a plurality of through holes having a desired shape, with high precision. In addition, it is possible to accurately form a plurality of openings simultaneously without the need for an expensive optical system. Rather the precision with which the mask is formed determines the precision of the through holes formed in the ceramic green sheet. This also makes it possible to use a relatively low precision positioning table, which reduces the cost equipment.
According to the present invention, by applying a laser beam to a ceramic green sheet as the sheet is moved, through holes can be successively punched in the ceramic green sheet, which provides a more suitable manufacturing method. In addition, using a pulsed laser can suppress an increase in the temperature of the ceramic green sheet as it is processed.
According to the present invention, by using a galvanomirror to reflect a laser beam emitted from a laser source to a mask, the laser beam can scan a broad portion of the ceramic green sheet without the need to move the ceramic green sheet. As a result, the overall movement of the ceramic green sheet is minimized, which enables an improvement in processing efficiency.
According to the present invention, by forming both through holes and positioning holes in the same process, their relative positional precision is be increased, which increases the precision with which conductor patterns can be formed in subsequent processes. Also, the need for forming positioning-holes is eliminated, which can shorten production time.
According to the present invention, by using a mask material having high reflectance to a laser beam, a CO2 laser, and a ceramic green sheet with a carrier film having one surface covered with a resin carrier film, the unnecessary portion of the laser beam (those portions of the beam which do not correspond to the through hole transmissive portions) emitted onto the mask is completely shielded by the mask, and the portions of the laser beams which passes through hole transmissive portions and transmissive alignment portions, are efficiently absorbed, so that through holes and positioning hole openings having a desired shape can be formed with higher precision.